Method and system for viewing a rendered volume

ABSTRACT

A technique is provided for viewing a series of volumes that may be rendered using visualization techniques incorporating one or more weighting functions, such as depth-dependent weighting functions. In one aspect, the viewing technique may utilize such weighting functions to vary the volume of interest in a series of volume renderings. In addition, the viewing technique may provide for varying the viewing angle to minimize the effect of orientation specific artifacts in the displayed images.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of medical imaging,and more specifically to the field of tomosynthesis. In particular, thepresent invention relates to the visualization of reconstructed volumesfrom data acquired during tomosynthesis.

Tomosynthesis is an imaging modality that may be used in a medicalcontext to allow physicians and radiologists to non-invasively obtainthree-dimensional representations of selected organs or tissues of apatient. In tomosynthesis, projection radiographs, conventionally knownas X-ray images, are acquired at different angles relative to thepatient. Typically, a limited number of projection radiographs areacquired over a relatively small angular range. The projectionscomprising the radiographs generally reflect interactions between x-raysand the imaged object along the respective X-ray paths through thepatient and, therefore, convey useful data regarding internalstructures. From the acquired projection radiographs, athree-dimensional volumetric image representative of the imaged volumemay be reconstructed.

The reconstructed volumetric image may be reviewed by a technologist orradiologist trained to generate a diagnosis or evaluation based on suchdata. In such a medical context, tomosynthesis may providethree-dimensional shape and location information of structures ofinterest as well as an increased conspicuity of the structures withinthe imaged volume. Typically, the structures within the reconstructedvolumetric image, or within a slice, have a significantly highercontrast than in each of the respective projection images, i.e.,radiographs.

However, evaluating the three-dimensional volumetric image may posechallenges in clinical practice. For example, viewing the volumetricimage slice by slice may require viewing forty to sixty slices or more.Therefore, small structures present in a single slice may be easilymissed. Moreover, the three-dimensional position and shape information,in particular the depth information (i.e., essentially in the directionof projection for the data acquisition), is only implicitly contained inthe stack of slices, with the “depth” of a structure that is locatedwithin a given slice being derived from the position of that slicewithin the full slice sequence or the volumetric image.

To address these problems, three-dimensional volume visualization orvolume rendering may be employed. These visualization techniques attemptto show the full three-dimensional volumetric image simultaneously, withthe location and shape information being conveyed mainly through changesin view angle, i.e., perspective. In addition, volume visualization maybe enhanced by including an occlusion effect, which hides (or partiallyhides) structures that are located behind other structures, depending onthe view angle.

However, one drawback of many volume rendering methods is an associatedloss of contrast, which may more than offset gains in contrast achievedby the three-dimensional reconstruction process. This problem typicallyoccurs when showing the full volume from a view angle requires some typeof averaging of values of the volumetric image for a range of depths. Asa result, the perceived contrast of a small structure may besignificantly smaller in the rendered image than in the originalprojection image data set. In addition, if a structure of interest isnot located “close to the viewpoint,” i.e., in front of most otherstructures, as seen from the viewpoint, occlusion effects may furtherdiminish the contrast of the structure, or even hide it completely. Thisproblem may be addressed by visualizing, i.e., rendering, only theregion or volume of interest within the volumetric image. Thistechnique, however, requires either a priori knowledge of the volume ofinterest or an intelligent way of continuously adjusting the volume ofinterest during the volume rendering process to allow the visualizationof any subvolume of the full reconstructed volumetric image. A techniquefor visualizing three-dimensional tomosynthesis data that provides goodvisualization of the three-dimensional context, i.e., localization andspace information, without reducing contrast may, therefore, bedesirable. Similarly, viewing modes which take advantage of theproperties of such visualization techniques may also be desirable.

BRIEF DESCRIPTION OF THE INVENTION

The present technique provides a novel approach to visualizingthree-dimensional data, referred to as volumetric images, such as dataprovided by tomosynthesis imaging systems. In particular, the presenttechnique provides for the use of weighting functions, such asdepth-dependent weighting functions, in the determination of pixelvalues in the volume rendered image from voxel values in the volumetricimage. Weighting functions may modify the voxel value itself and/orother modifiers of the voxel value, such as opacity functions.Furthermore, the technique provides for novel viewing modes, such asvarying the volume of interest via the weighting function or functions.Other novel viewing modes may include varying the view angle to reduceartifacts attributable to the scan trajectory and simultaneouslydisplaying different volume renderings with common reference image databut different perspectives.

In accordance with one aspect of the technique, a method is provided forviewing a sequence of rendered volume images. In accordance with thisaspect, a volume of interest comprising reconstructed three-dimensionalvolumetric image data is selected. A reference depth may be varied whichaffects the operation of one or more weighting functions used to rendervolume images representing the volume of interest.

In accordance with another aspect of the technique, a method is providedfor viewing a sequence of rendered volume images. In accordance withthis aspect, a volume comprising reconstructed three-dimensionalvolumetric image data is selected. A view angle for rendering volumeimages of the volume may then be varied such that the visual presence ofone or more orientation specific image artifacts is reduced. Systems andcomputer programs that afford functionality of the type defined by theseaspects are also provided by the present technique.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome apparent upon reading the following detailed description and uponreference to the drawings in which:

FIG. 1 is a diagrammatical view of an exemplary imaging system in theform of a tomosynthesis imaging system for use in providing volumetricimages and producing visualizations of the volumetric images inaccordance with aspects of the present technique; and

FIG. 2 depicts an exemplary volumetric image and the aspects of thevolumetric image as they relate to three-dimensional visualization.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In the field of medical imaging, various imaging modalities may beemployed to non-invasively examine and/or diagnose internal structuresof a patient using various physical properties. One such modality istomosynthesis imaging which utilizes a limited number of projectionradiographs, typically twenty or less, each acquired at a differentangle relative to a patient. The projection radiographs may then becombined to generate a volumetric image representative of the imagedobject, i.e., a three-dimensional set of data that providesthree-dimensional context and structure for the volume of interest. Thepresent technique addresses visualization issues that may arise in thedisplay of volumetric images provided by tomosynthesis imaging. Inparticular, the present technique allows for the incorporation ofweighting into the visualization process and for various viewing modesthat may benefit from such weighting.

An example of a tomosynthesis imaging system 10 capable of acquiringand/or processing image data in accordance with the present technique isillustrated diagrammatically in FIG. 1. As depicted, the tomosynthesisimaging system 10 includes an X-ray source 12, such as an X-ray tube andassociated components, e.g., for support and filtering. The X-ray source12 may be moved within a constrained region. As one of ordinary skill inthe art will appreciate, the constrained region may be arcuate orotherwise three-dimensional. For simplicity, however, the constrainedregion is depicted and discussed herein as a plane 14 within which thesource 12 may move in two-dimensions. Alternatively, a plurality ofindividually addressable and offset radiation sources may be used.

A stream of radiation 16 is emitted by the source 12 and passes into aregion in which a subject, such as a human patient 18, is positioned. Aportion of the radiation 20 passes through or around the subject andimpacts a detector array, represented generally at reference numeral 22.The detector 22 is generally formed by a plurality of detector elements,generally corresponding to pixels, which produce electrical signals thatrepresent the intensity of the incident X-rays. These signals areacquired and processed to reconstruct a volumetric image representativeof the features within the subject. A collimator may also be present,which defines the size and shape of the X-ray beam 16 that emerges fromthe X-ray source 12.

Source 12 is controlled by a system controller 24 which furnishes bothpower and control signals for tomosynthesis examination sequences,including positioning of the source 12 relative to the patient 18 andthe detector 22. Moreover, detector 22 is coupled to the systemcontroller 24, which commands acquisition of the signals generated inthe detector 22. The system controller 24 may also execute varioussignal processing and filtration functions, such as for initialadjustment of dynamic ranges, interleaving of digital image data, and soforth. In general, system controller 24 commands operation of theimaging system 10 to execute examination protocols and to acquire theresulting data.

In the exemplary imaging system 10, the system controller 24 commandsthe movement of the source 12 within the plane 14 via a motor controller26, which moves the source 12 relative to the patient 18 and thedetector 22. In alternative implementations, the motor controller 26 maymove the detector 22, or even the patient 18, instead of or in additionto the source 12. Additionally, the system controller 24 may include anX-ray controller 28 to control the activation and operation of the X-raysource 12. In particular, the X-ray controller 28 may be configured toprovide power and timing signals to the X-ray source 12. By means of themotor controller 26 and X-ray controller 28, the system controller 24may facilitate the acquisition of radiographic projection images atvarious angles through the patient 18.

The system controller 24 may also include a data acquisition system 30in communication with the detector 22. The data acquisition system 30typically receives data collected by readout electronics of the detector22, such as sampled analog signals. The data acquisition system 30 mayconvert the data to digital signals suitable for processing by aprocessor-based system, such as a computer 36.

The computer 36 is typically coupled to the system controller 24. Thedata collected by the data acquisition system 30 may be transmitted tothe computer 36 for subsequent processing, reconstruction and volumerendering. For example, the data collected from the detector 22 mayundergo correction and pre-processing at the data acquisition system 30and/or the computer 36 to condition the data to represent the lineintegrals of the attenuation coefficients of the scanned objects. Theprocessed data, commonly called projections, may then be used as inputto a reconstruction process to formulate a volumetric image of thescanned area. Once reconstructed, the volumetric image produced by thesystem of FIG. 1 reveals an internal region of interest of the patient18 which may be used for diagnosis, evaluation, and so forth. Computer36 may also compute volume rendered images of the reconstructedvolumetric image, which may then be displayed on display 42. In analternative embodiment, some functions of the computer 36 may be carriedout by additional computers (not shown), which may include specifichardware components, such as for fast three-dimensional reconstructionor volume rendering.

The computer 36 may comprise or communicate with memory circuitry thatcan store data processed by the computer 36 or data to be processed bythe computer 36. It should be understood that any type of computeraccessible memory device capable of storing the desired amount of dataand/or code may be utilized by such an exemplary system 10. Moreover,the memory circuitry may comprise one or more memory devices, such asmagnetic or optical devices, of similar or different types, which may belocal and/or remote to the system 10. The memory circuitry may storedata, processing parameters, and/or computer programs comprising one ormore routines for performing the processes described herein.

The computer 36 may also be adapted to control features enabled by thesystem controller 24, i.e., scanning operations and data acquisition.Furthermore, the computer 36 may be configured to receive commands andscanning parameters from an operator via an operator workstation 40which may be equipped with a keyboard and/or other input devices. Anoperator may thereby control the system 10 via the operator workstation40. Thus, the operator may observe acquired projection images,reconstructed volumetric images and other data relevant to the systemfrom computer 36, initiate imaging, and so forth.

A display 42 coupled to the operator workstation 40 may be utilized toobserve the reconstructed volumetric images and to control imaging.Additionally, the images may also be printed by a printer 44 that may becoupled to the operator workstation 40. The display 42 and printer 44may also be connected to the computer 36, either directly or via theoperator workstation 40. Further, the operator workstation 40 may alsobe coupled to a picture archiving and communications system (PACS) 44.It should be noted that PACS 44 may be coupled to a remote system 46,radiology department information system (RIS), hospital informationsystem (HIS) or to an internal or external network, so that others atdifferent locations may gain access to the image and to the image data.

It should be further noted that the computer 36 and operator workstation40 may be coupled to other output devices that may include standard orspecial purpose computer monitors and associated processing circuitry.One or more operator workstations 40 may be further linked in the systemfor outputting system parameters, requesting examinations, viewingimages, and so forth. In general, displays, printers, workstations, andsimilar devices supplied within the system may be local to the dataacquisition components, or may be remote from these components, such aselsewhere within an institution or hospital, or in an entirely differentlocation, linked to the image acquisition system via one or moreconfigurable networks, such as the Internet, virtual private networks,and so forth.

Once reconstructed and combined, the volumetric image data generated bythe system of FIG. 1 reveal the three-dimensional spatial relationshipand other characteristics of internal features of the patient 18. Toconvey useful medical information contained within the image data, avisualization technique may be employed to represent aspects of theimage data to a technologist or radiologist. For example, in traditionalapproaches to diagnosis of medical conditions, a radiologist mightreview one or more slices of the volumetric image data, either on aprinted medium, such as might be produced by the printer 44, or on thedisplay 42. Features of interest might include nodules, lesions, sizesand shapes of particular anatomies or organs, and other features thatmay be discerned in the volumetric image data based upon the skill andknowledge of the individual practitioner.

Other analyses may be based upon a volume rendering or visualizationtechnique that allows for the simultaneous viewing of the fullthree-dimensional data set. Such techniques allow three-dimensionallocation and shape information to be conveyed in a more natural andintuitive way than in slice viewing, though with a possible reduction inthe contrast of small structures. In particular, depth informationwithin the rendered or visualized volume may be conveyed through theperceived relative motion of structures when changing perspectives,i.e., view angles. Furthermore, occlusion effects may be introduced toconvey the depth ordering of structures at a particular perspective,further enhancing the perception of depth. In conjunction with occlusioneffects, the degree of transparency of structures may be adjustable toallow control of the depth of penetration when viewing the image data.In addition to varying perspective to facilitate the perception ofdepth, the volume of interest may also be adjusted to exclude structuresfrom the rendered image and/or to optimize the contrast of smallstructures in the rendered image.

For example, referring to FIG. 2, a volume rendered image for areconstructed volumetric image 49 may be generated by specifying a viewangle 52, associated with the desired viewpoint, and an image plane 50,which may or may not be parallel to the respective slices of thevolumetric image data. The intensity values associated with theintersection of a ray 54 and the volume of interest 56 may then beprojected onto a corresponding pixel 58 of the image plane 50. Inparticular, the intensity value of a pixel 58 in the rendered image maybe derived from some functional relationship of the intensity values, orother signal properties, of the reconstructed volumetric image 49 atlocations along the ray. As one of ordinary skill in the art willappreciate, the ray 54 is associated with a particular viewingdirection, which may determine such things as the ordering of values inan associated volume rendered image.

For example, the intensity or gray scale value, r, of a pixel in therendered volume may be given by the equation: $\begin{matrix}{{r = {\int_{a}^{b}{{{w(t)} \cdot {\mathbb{e}}^{- {\int_{0}^{t}{{o{(s)}}\;{\mathbb{d}s}}}}}{\mathbb{d}t}}}},} & (1)\end{matrix}$where both integrals are path integrals of values that are functions ofthe values, v, of the three-dimensional volumetric image data set at thecorresponding locations, i.e., w(t) is not a function of, t, but of,v(t). Thus, the opacity, o, and the value w depend on the correspondingvolumetric image values, v, their functional relationship being definedby suitable transfer functions. While FIG. 2 suggests that equation (1)is parameterized in terms of the depth, t, in standard volume renderingt represents the path length along the considered ray 54. For small viewangles, however, these parameterizations are essentially equivalent.

As one of ordinary skill in the art will appreciate, equation (1)generates a pixel value in the rendered image where the contribution ofeach voxel value along the ray 54 is weighted by the opacity, o,associated with all voxels in front of it. Indeed, the opacity term inequation (1), when discretized, introduces a multiplicative weightingof: e^(−(a+b))=e^(−a)·e^(−b). Therefore, a volume-rendered image,r(x,y), may be created by evaluating equation (1) for all rays 54,defined by the associated image pixel 58 coordinates (x, y),corresponding to the specified view point, view angle 52 and image plane50.

In practice, various visualization geometries may be employed. Forexample, in parallel projection geometry all rays 54 are assumed to beparallel and the view angle 52 is the angle of the rays 54 through thevolumetric image 49. Conversely, in cone beam geometry, the rays 54 areassumed to all go through a common point and the view angle 52 can bedefined as the angle of that viewpoint with respect to some referencepoint in the image. The present technique may be utilized with parallelprojection or cone beam geometries as well as with other renderinggeometries that may be employed. In addition to the rendering processdescribed, other data conditioning and/or normalization processes may beperformed, such as normalization by total pathlength through the volumeof interest 56, |b−a|, which do not affect the implementation of thepresent technique.

The preceding discussion and equation (1) generally relate to thevisualization technique known as composite ray-casting. Special cases ofequation (1) may correspond to other visualization techniques, however.For example, a zero-opacity case generally corresponds to what is knownas an X-ray projection viewing mode. Similarly, if the volume ofinterest is defined by depths a and b that are close, such as where|b−a| is constant, the visualization mode is known as thick sliceviewing. In cases where the interval [a,b] encompasses only a singleslice, i.e., slice-by-slice viewing, the visualization methodcorresponds to a slice viewing mode. Other visualization modes may alsobe utilized, such as maximum or minimum intensity projection. As notedabove, these various visualization techniques may present difficultieswith regard to small structures of interest or may not show the fullthree-dimensional context that would facilitate the interpretation ofthe volumetric image, 49. In particular, small structures may have poorcontrast when visualized by these and other techniques known in the art.As a result, the small structures of interest may be easy to miss withinthe visualized data set. In addition, three-dimensional context may beinsufficient for easy interpretation in some viewing modes, such as inslice-by-slice viewing mode. Existing volume rendering methods typicallyoffer only sub-optimal compromises between visualization of thethree-dimensional context and contrast of small structures.

I. Volume Rendering Approaches Incorporating Weighting

To improve visualization, a weighting component may be included in thedetermination of pixel intensity in the rendered image. For example,weighting functions may allow for depth dependence in the determinationof intensity values and/or opacity values of voxels of the reconstructedvolumetric image 49, which will in turn impact the pixel values in therendered image. Furthermore, the weighting functions may allow tradeoffs to be made with regard to image quality, typically the contrast, ofa structure of interest and the three-dimensional context associatedwith the structure. As a result, the operator may increase the contrastof a structure of interest while still maintaining some acceptable orsuitable amount of associated context.

A. Zero-Opacity Approaches

For example, equation (1), without the opacity component, may bemodified in the following manner: $\begin{matrix}{{r = {\int_{a}^{b}{{w(t)}\;{g(t)}\mspace{11mu}{\mathbb{d}t}}}},} & (2)\end{matrix}$where g is a depth-dependent weighting function. The weighting functionallows a compromise between good image contrast and goodthree-dimensional perception to be reached. For example, weightingfunctions which may be employed include g(t)=1−α·|t−t₀| org(t)=e^(−α·|t−t) ⁰ ^(|), with a≦t₀≦b. These weighting functions allow usto focus on the slice at depth t₀, while still showing thethree-dimensional context, but with reduced intensity. Alternatively,the weighting function may be specified to be non-symmetric (relative tot₀) such as to put more emphasis on structures “in front of”, or“behind” the slice at depth t₀. In one embodiment, the weightingfunction allows structures at depth t₀ to be viewed in conjunction withsome of the three-dimensional context from “behind” that plane, whilestill maintaining a good contrast of structures at height t₀. This maybe accomplished by choosing one of the preceding weighting functions, g,defined such that g(t)=0 for t<t₀. Other weighting functions may be usedas well, such as to weight one slice (e.g., at depth t₀) or multipleslices, more than other slices. In some cases the weighting function maybe set to zero outside of a given interval, thus further focusing therendering to an even smaller volume of interest.

As one of ordinary skill in the art will appreciate, the depth t₀, asused herein, may be associated with the depth of a slice, as indicatedin the preceding discussion. In addition, t₀ may be associated withother planes or hypersurfaces within the volume of interest 56. Forexample, t₀ may denote the distance from the viewpoint, in which caset=t₀ describes a part of a spherical surface, or t₀ may denote thedistance from the surface of the imaged object.

B. Maximum Intensity Projection Approaches

Equation (2) can also be used as an alternative representation of amaximum intensity projection (MIP) technique in which the weightingfunction, g, is a delta impulse that is data driven, i.e., the deltaimpulse may be specified at the location of the maximum intensity alongthe ray 54. Alternatively, other weighting functions, g, may beemployed, such as rectangular pulses, or the previously discussedweighting functions may be employed such that the “location” t₀ isdetermined, for example, by the location of the maximum intensity valuealong the ray 54. In addition, instead of using the single maximum valuealong the ray 54, the two (or more) highest values along the ray 54 maybe used instead. For example, some function of these two values may beassigned as the pixel value in the rendered image. Since the intensityprofiles along rays are typically “smooth”, it may be desirable todecompose the intensity profile into several “modes” for processing,such as by interpreting the intensity profile as a linear combination ofGaussians, and to choose the amplitude of the two (or more) largestGaussians to be combined into the pixel value of the rendered image.Another weighted generalized equation may be expressed as:r=max _(tε[a,b])(w(t) ·g(t)),  (3)which has similar characteristics as MIP but favors values close to acertain height t₀ due to the additional weighting function, g. Whilemaximum intensity projection techniques have been discussed, one skilledin the art will readily appreciate that the described approaches may bereadily and easily adapted as minimum intensity projection techniques ifdesired.C. Opacity Approaches

Equation (2) may be modified to include an opacity weighting function,h, to obtain: $\begin{matrix}{r = {\int_{a}^{b}{{{w(t)} \cdot {g(t)} \cdot {\mathbb{e}}^{- {\int_{0}^{t}{{{o{(s)}} \cdot {h{(s)}}}\;{\mathbb{d}s}}}}}{{\mathbb{d}t}.}}}} & (4)\end{matrix}$Either or both of the weighting functions, g and h, may be formulated inaccordance with the preceding discussions of the weighting function g orin accordance with different weighting priorities. The addition of aweighting factor, h, for the occlusion term allows structures to bedifferentially occluded based on their height. For example, structuresthat are at or above a depth t₀ may be more lightly occluded, or notoccluded at all, while structures below the depth t₀ may be occluded toa greater extent. In this manner, a clearly perceptible occlusion effectcaused by structures in the “foreground” may be achieved while stillmaintaining a significant penetration throughout the volume.

An example of an implementation that may provide good penetrationthrough a volume includes an opacity weighting function, h, that is zerofor t<t₀, has its maximum at t=t₀, and that quickly falls off to zerofor t>t₀. This opacity weighting function may be used in conjunctionwith an intensity weighting function, g, that is either small for t<t₀,to provide some three-dimensional context in the foreground, or zero fort<t₀, and that falls off more slowly relative to h so as to allowsufficient penetration of the volume.

In another implementation, slices or other portions of the volumetricimage that are located “behind” and occluded by other slices may becontrast-enhanced, such as by some type of high-pass filtering. In thismanner, the three-dimensional perception through the relative motion andthe occlusion of different structures is maintained, but the visibilityof occluded structures is better preserved. This approach may be furthermodified to increase or vary the contrast enhancement based on depth.For example, contrast enhancement may be increased as depth increases.

While the preceding discussion provides examples in the form of variousequations, one skilled in the art will readily appreciate that suchequations are merely intended to be illustrative and not exclusive.Indeed, other equations may also be used to achieve similar effects andare considered to be within the scope of the present technique.Furthermore, though equations (2)–(4) are represented in integralnotation for brevity and simplicity, computational implementations ofthe calculations expressed by equations (2)–(4) may be by discreteapproximation of these equations.

In addition, though the present discussion focuses on gray-scale images,the present technique is equally applicable to color images. Forexample, a weighting function, g and/or h, may associate a differentcolor, as opposed to gray-scale value, with different depths.Applications in color visualization are also considered to be within thescope of the present technique.

II. Viewing Modes Incorporating Weighting Functions

The approaches discussed above generally address the generation of asingle rendered image from a three-dimensional volumetric image dataset. However, the benefit of volume rendering may be greatly improvedthrough viewing a sequence of rendered images that vary in theirviewpoint, view angle, and/or in the volume of interest 56 rendered. Inparticular, the perceived motion of different structures in a sequenceof volume rendered images is a primary contributor to depth perceptionand to an intuitive understanding of the position and shape ofthree-dimensional structures within the reconstructed volumetric image49. The use of weighting functions in the volume rendering process mayalso have implications for the potential viewing modes used in viewingthe resulting sequence of images.

A. Variation of the Volume of Interest

For example, in slice viewing the volume of interest 56, as defined bythe start height, a, and the end height, b, is continuously modified toscan through the whole stack of slices. In generalized approachesdescribed herein, the volume of interest 56 may be defined relative to avarying reference height t₀ (or vice versa). Alternatively, since theweighting functions g and h can be modified so as to control the startand end-height of the volume of interest 56 as well, it may besufficient to continuously vary the reference height t₀ that controlsthe “location” or focus of the intensity and opacity weightingfunctions, while the volume of interest 56, as defined by the start andend heights a and b, encompasses the full reconstructed volume. In thismanner, the boundaries of the volume of interest 56 may be defined interms of depth by controlling the selection of a and b or by having acorresponding cut-off or smooth drop-off in the weighting functions gand h at the corresponding start and end heights.

One can also have lateral boundaries within a volumetric image in the xand y directions which are either hard boundaries or which areimplemented as a lateral, possibly smooth, drop-off in the weightingfunctions g and h. An approach based on weighting may involve thedefinition of the weighting functions, such as g and/or h, as functionsof three variables. By continuously varying the volume of interest 56 interms of depth and in terms of x and y, one may be able to scan thereconstructed volume in a “telescoping” or selective manner, i.e.,focusing on particular volumes of interest, as defined by x, y, anddepth, at will. For example, the weighting functions may be defined witha drop-off both in terms of depth as well as laterally. In such anapproach, a weighting function may be controlled by centering it arounda reference point within the volumetric image 49. The reference point,as one of ordinary skill in the art will appreciate, may be defined by adepth, t₀, as well as by x and y coordinates. By varying the coordinatesof the reference point, i.e., by “moving” the reference point, one alsomoves the weighting function and the corresponding cut-off or smoothdrop implemented by the weighting function in any of thethree-dimensions.

B. Variation of the Viewing Angle

As one of ordinary skill in the art will appreciate, a side view of thetomosynthesis data set generally exhibits relatively poor resolution.For this reason, a systematic variation of the view angle in x and y,such that the view angle 52 remains relatively small, is desirable. Forexample, in the so-called “tumble view,” the view angle describes acircle relative to the x,y plane, where the center of the circle isaligned with the center of the slices of the reconstructed volumetricimage 49 or volume of interest 56. The radius of the circle willgenerally be a function of the depth resolution of the volumetric imagedata set, which in turn may be a function of the tomosynthesisacquisition geometry.

In some circumstances, artifacts in the reconstructed volumetric image49 may have a preferred orientation as a function of the acquisitiongeometry, i.e., the path of the source 12 during the acquisition of thetomosynthesis data set. In these circumstances, other trajectories forthe view angle may be desirable to facilitate the apprehension of thethree-dimensional structure while minimizing the impact of theseorientation dependent artifacts on the visualization process. Inparticular, other trajectories for the view angle may reduce theoccurrence of, the size of, or the intensity of such orientationspecific image artifacts. For example, in linear tomosynthesis, wherethe X-ray source 12 moves along a linear trajectory, the use of anelliptical trajectory for the view angle, where the long axis of theellipse is aligned with the scanning trajectory of the X-ray source 12,may be beneficial.

C. Combinations of Volume of Interest and Viewing Angle

If the overall volume is too thick to allow a meaningful visualizationof the full three-dimensional volume at one time, it may be desirable tovary both the view angle and the volume of interest 56 to improve thedisplay image quality. For example, a spiral tumble or circular tumblewith the depth location of the volume of interest 56 changing as theview angle changes may be desirable for thick volumes. In this example,the variation of the depth location of the volume of interest 56, as afunction of the view angle, may be constrained such that a 180 degree,i.e., a half-circle, sweep of the view point is associated with movementof the depth location of the volume of interest 56 of less than thethickness of the volume of interest 56. Such a constraint allows everystructure to be seen from at least two opposite sides.

D. Simultaneous Display of Images

To better convey the three-dimensional context of the volume, it may beadvantageous to display different volume renderings of the same volumein different panes or windows of the display 42 or on separate butproximate displays 42. For example, it may be useful to show a volumerendering from a forward viewpoint and from a backward viewpoint of thesame volume of interest 56 or of different volumes of interest 56 thatoverlap. In such a context, a ray 54 through the center of the volume ofinterest may be common to both the forward and backward viewpoint,differing only in the ordering of the values along the ray 54. In suchan example, both volumes of interest 56 and the associated transferfunctions may essentially be “mirror images” of one another with respectto some reference height t₀. By simultaneous display of such images,both images can show the same region of the volume in focus whileproviding three-dimensional context in front of as well as behind thisregion. Changing the view angle may automatically update the view anglefor both views. In addition, by sweeping the height t₀ through thevolume during viewing, the full volumetric image 49 can be scanned.

In another example, one can have a central rectangular pane or window,with four adjacent panes arranged around the periphery of the centralpane. The central pane may show a single rendered image of a volume ofinterest 56 while the other panes show the same volume of interest 56from a view angle that is offset by a constant, but adjustable, anglefrom the view angle used to generate the center image. The direction ofthe offset may be conveyed by the relative location of a peripheral paneto the central pane, i.e., a pane to the right of center may show animage corresponding to a view angle that is offset to the right, and soforth. In this example, the volume of interest 56 may also be sweptthrough the entire volumetric image data set during viewing so that thefull volume may be observed. Similarly, different rendered images may becolor coded and superimposed on a single display, as opposed toside-by-side or proximate display.

Alternatively, it may be of interest to simultaneously display volumerenderings of the one or more volumes of interest 56 in which variousdisplay and/or rendering variables or functions are varied. For example,the same volume of interest 56 may be simultaneously displayed but withdifferent intensity and/or occlusion weighting functions. Alternatively,one or more weighting functions may be constant or identical, butrespective transfer functions, such as for determining intensity orocclusion, may be different for the simultaneously displayed renderings.To compare the simultaneously displayed renderings solely on basis ofthe varied parameters, the volume of interest 56 may be displayed at thesame view angle, view geometry, and so forth. Such an approach may beuseful for distinguishing or comparing characteristics or structures inthe data that may be differentiated based on the varied parameter. Forexample, some structures of interest may be more easily discerned in arendering generated using a first set of weighting functions while otherstructures of interest in the same volume of interest 56 may be moreeasily discerned in a rendering generated using a second set ofweighting functions.

The invention may be susceptible to various modifications andalternative forms, and specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. A method for viewing a sequence of rendered volume images,comprising: selecting a volume of interest comprising reconstructedthree-dimensional volumetric image data; and varying a reference depthaffecting the operation of one or more weighting functions used torender volume images representing the volume of interest.
 2. The method,as recited in claim 1, wherein the volume of interest encompasses anentire reconstructed volume.
 3. The method, as recited in claim 1,wherein varying the reference depth affects the operation of at leastone weighting function by changing the location of a cut-off determinedfrom the weighting function.
 4. The method, as recited in claim 1,wherein varying the reference depth affects the operation of at leastone weighting function by changing the location of a smooth drop-offdetermined from the weighting function.
 5. The method, as recited inclaim 1, comprising the step of varying a coordinate in an x,y-planeaffecting the operation of the one or more weighting functions, whereinvarying the coordinate in the x,y-plane affects the rendering of volumeimages in the x,y plane and varying the reference depth affects therendering of volume images in a direction perpendicular to thex,y-plane.
 6. The method, as recited in claim 5, wherein varying thecoordinate in the x,y-plane affects the operation of at least oneweighting function by changing the location of a cut-off determined fromthe weighting function.
 7. The method, as recited in claim 5, whereinvarying the at least one coordinate in the x,y-plane affects theoperation of at least one weighting function by changing the location ofa smooth drop-off determined from the weighting function.
 8. The method,as recited in claim 1, wherein at least one weighting function modifiesan intensity value.
 9. The method, as recited in claim 1, wherein atleast one weighting function modifies an opacity value.
 10. A computerprogram, provided on one or more computer readable media, for viewing asequence of rendered volume images, comprising: a routine for selectinga volume of interest comprising reconstructed three-dimensionalvolumetric image data; and a routine for varying a reference depthaffecting the operation of one or more weighting functions used torender volume images representing the volume of interest.
 11. Thecomputer program, as recited in claim 10, wherein the routine forvarying affects the operation of at least one weighting function bychanging the location of a cut-off determined from the weightingfunction.
 12. The computer program, as recited in claim 10, wherein theroutine for varying affects the operation of at least one weightingfunction by changing the location of a smooth drop-off determined fromthe weighting function.
 13. The computer program, as recited in claim10, comprising: a routine for varying a coordinate in an x,y-planeaffecting the operation of the one or more weighting functions, whereinvarying the reference depth affects the rendering of volume images in afirst direction and varying the coordinate in the x,y-plane affects therendering of volume images in a plane perpendicular to the firstdirection.
 14. The computer program, as recited in claim 13, wherein theroutine for varying the coordinate in the x,y-plane affects theoperation of at least one weighting function by changing the location inthe x,y-plane of a cut-off determined from the weighting function. 15.The computer program, as recited in claim 13, wherein the routine forvarying the coordinate in the x,y-plane affects the operation of atleast one weighting function by changing the location in the x,y-planeof a smooth drop-off determined from the weighting function.
 16. Thecomputer program, as recited in claim 10, wherein at least one weightingfunction modifies an intensity value.
 17. The computer program, asrecited in claim 10, wherein at least one weighting function modifies anopacity value.
 18. A tomosynthesis imaging system, comprising: an X-raysource configured to emit a stream of radiation through a volume ofinterest from different position relative to the volume of interest; adetector array comprising a plurality of detector elements, wherein eachdetector element may generate one or more signals in response to therespective streams of radiation; a system controller configured tocontrol the X-ray source and to acquire the one or more signals from theplurality of detector elements; a computer system configured to receivethe one or more signals, to reconstruct a three-dimensional volumetricimage data set from the one or more signals, and to render a series ofvolumes from the three-dimensional volumetric image data set by varyinga reference depth affecting the operation of one or more weightingfunctions used to render each volume; and an operator workstationconfigured to display the series of volumes.
 19. The tomosynthesisimaging system, as recited in claim 18, wherein varying the referencedepth affects the operation of at least one weighting function bychanging the location of a cut-off determined from the weightingfunction.
 20. The tomosynthesis imaging system, as recited in claim 18,wherein varying the reference depth affects the operation of at leastone weighting function by changing the location of a smooth drop-offdetermined from the weighting function.
 21. The tomosynthesis imagingsystem, as recited in claim 18, wherein the computer system is furtherconfigured to render the series of volumes by varying a coordinate in anx,y-plane affecting the operation of the one or more weightingfunctions, wherein varying the coordinate in the x,y-plane affects therendering of volumes in the x,y-plane and varying the reference depthaffects the rendering of volumes in a direction perpendicular to thex,y-plane.
 22. The tomosynthesis imaging system, as recited in claim 21,wherein varying the coordinate affects the operation of at least oneweighting function by changing the location of a cut-off determined fromthe weighting function.
 23. The tomosynthesis imaging system, as recitedin claim 21, wherein varying the coordinate affects the operation of atleast one weighting function by changing the location of a smoothdrop-off determined from the weighting function.
 24. The tomosynthesisimaging system, as recited in claim 18, wherein at least one weightingfunction modifies an intensity value.
 25. The tomosynthesis imagingsystem, as recited in claim 18, wherein at least one weighting functionmodifies an opacity value.
 26. A tomosynthesis imaging system,comprising: means for selecting a volume of interest comprisingreconstructed three-dimensional volumetric image data; and means forvarying a reference depth affecting the operation of one or moreweighting functions used to render volume images representing the volumeof interest.
 27. A method for viewing a sequence of rendered volumeimages, comprising: selecting a volume comprising reconstructedthree-dimensional volumetric image data; and varying a view angle forrendering volume images of the volume such that the visual presence ofone or more orientation specific image artifacts is reduced.
 28. Themethod, as recited in claim 27, wherein varying the view angle comprisesvarying the view angle along an elliptical trajectory such that the longaxis of the elliptical trajectory corresponds to a linear scanningtrajectory of an X-ray source.
 29. A computer program, provided on oneor more computer readable media, for viewing a sequence of renderedvolume images, comprising: a routine for selecting a volume comprisingreconstructed three-dimensional volumetric image data; and a routine forvarying a view angle for rendering volume images of the volume such thatthe visual presence of one or more orientation specific image artifactsis reduced.
 30. The computer program, as recited in claim 29, whereinthe routine for varying the view angle varies the view angle along anelliptical trajectory such that the long axis of the ellipticaltrajectory corresponds to a linear scanning trajectory of an X-raysource.
 31. A tomosynthesis imaging system, comprising: an X-ray sourceconfigured to emit a stream of radiation through a volume of interestfrom different position relative to the volume of interest; a detectorarray comprising a plurality of detector elements, wherein each detectorelement may generate one or more signals in response to the respectivestreams of radiation; a system controller configured to control theX-ray source and to acquire the one or more signals from the pluralityof detector elements; a computer system configured to receive the one ormore signals, to reconstruct a three-dimensional volumetric image dataset from the one or more signals, and to render a series of volumes fromthe three-dimensional volumetric image data set by varying a view anglesuch that the visual presence of one or more orientation specific imageartifacts is reduced in the series of volumes; and an operatorworkstation configured to display the series of volumes.
 32. Thetomosynthesis imaging system, as recited in claim 31, wherein thecomputer system is configured to vary the view angle along an ellipticaltrajectory such that the long axis of the elliptical trajectorycorresponds to a linear scanning trajectory of the X-ray source.
 33. Atomosynthesis imaging system, comprising: means for selecting a volumecomprising reconstructed three-dimensional volumetric image data; andmeans for varying a view angle for rendering volume images of the suchthat the visual presence of one or more orientation specific imageartifacts is reduced.